1. Field of the Invention
This invention relates to an electronic linear scale, and more particularly, to electronic linear scales employing inductively coupled transducer elements.
2. Description of Related Art
Electronic linear scales are common in the manufacturing industry and wherever high precision measurements are required. Linear scales are often affixed to equipment such as drill presses, lathes, saws, numerically controlled and automated machinery. The linear scale allows these machines to produce goods with a high level of precision.
These scales have used transducers based on optical systems, magnetic scales, inductive transformation, and capacitive devices.
Conventional linear scales have used optical technology with incremental and absolute measurement systems. Problems with this system include a relatively high power requirement, high manufacturing cost due to the precise mechanical tolerances required, and sensitivity to contaminants. To make a measurement, the optical system used a self-contained lamp directed at an optical scale and transmitted through (or reflected) to an optical receiver. The lamp involved in this system usually consumes more energy than is practicably provided by conventional batteries. This reduces the portability and ease of installation of the linear scale. Because the lamp and the optical receiver traverse the scale during a measurement, this linear scale requires a precise guiding track to guide and support the optical components. This track must to constructed with close tolerances and the optical receiver must be precisely aligned, so that the image reflected off the optical scale is focused onto the optical receiver. Due to the close assembly tolerances required, the bearing system used in the device, and the cost of the optical-grade components, the optical systems have high manufacturing costs. Additionally, the effects of wear or poor alignment significantly degrade the performance.
Capacitive transducers draw very little current. Therefore, when used in battery-powered measurement devices, they provide portability and ease of installation. Capacitive transducers operate under a parallel plate capacitor model. Within the capacitive transducer, a transmitter electrode and a receiver electrode are mounted on or in a slide. The transmitter electrode is connected to appropriate signal generating circuitry. The receiver electrode is connected to appropriate read circuitry.
The slide moves along a scale. The scale includes a plurality of spaced-apart signal electrodes, which extend along the length of the scale. As the slide moves relative to the scale, the transmitter and receiver electrodes on the slide capacitively couple to the signal electrodes on the scale. The read circuitry determines the movement or position of the slide relative to the scale. The read circuitry compares the phase of at least one signal coupled to a receiver electrode with the phase of at least one signal coupled to a transmitter electrode.
The capacitive position transducer may be an incremental type transducer or may be an absolute position type transducer. In the incremental type capacitive position transducer, the read circuitry provides only an indication of relative movement from a known point. In the absolute position type capacitive position transducer, the read circuitry provides an indication of the absolute position between the slide and scale regardless of any knowledge of the initial position. Incremental and absolute position type position transducers are disclosed in U.S. Pat. Nos. 4,420,754 and 4,879,508.
These capacitive position transducers are sometimes used in dry, relatively clean environments, such as in inspection rooms or engineering offices. However, these capacitive position transducers are desirably usable in electronic linear scales to measure dimensions in machine shops and other relatively contamination-filled environments. In these environments, capacitive linear scales can become contaminated by particulate matter and fluids, such as metal particles, grinding dust, and cooling or cutting fluids. The liquid or particulate contaminants find their way between the signal electrodes on the scale and the transmitter and/or receiver electrodes on the slide. The contaminants alter the capacitance between the signal electrodes and the transmitter and/or receiver electrodes in a manner unrelated to the position of the slide relative to the scale. In general, contaminants between the signal electrodes and the transmitter electrodes and/or the receiver electrodes of a capacitive position transducer cause measurement errors through three different mechanisms. Primarily, the particulate or liquid may have a dielectric constant different from the dielectric constant of air. In this case, the capacitance between the signal electrodes and the transmitter/receiver electrodes sandwiching the contaminant will be greater than the capacitance between other ones of the signal and transmitter/receiver electrodes having the same relative geometry which do not have contaminants between them. As a result, the capacitive linear scale will not provide an accurate indication of the position of the slide relative to the scale.
Secondarily, the contaminants may have a relatively high conductivity. Normally, the signal and transmitter/receiver electrodes form an open circuit, such that no current flows between them. A conductive contaminant between the signal and transmitter or receiver electrodes closes this circuit. In particular, the contaminant forms the resistive element of an RC circuit. The time constant of the RC circuit is a function of both the conductivity of the contaminant and the capacitance between the signal electrode and the transmitter and/or receiver electrodes. When the time constant is relatively short, the amplitude of the signal may decay so rapidly that the conventional circuitry employed in capacitive position transducers cannot properly sense the signal.
Thirdly, electrically conductive particles between the signal electrode and the transmitter and/or receiver electrodes may alter the field extending between the signal electrode and the transmitter and/or receiver electrodes. This changes the capacitance between the signal electrode and the transmitter and/or receiver electrodes. Distortions in the electric field may also distort the signals between the signal electrode and the transmitter and/or receiver electrodes. As a result, the capacitive linear scale circuitry does not provide an accurate indication of the position of the slide relative to the scale.
U.S. Pat. No. 5,172,485 to Gerhard et al. describes one approach to minimizing the adverse effects of contaminants in capacitive position transducers. This approach comprises coating the electrodes with a thin layer of dielectric material. The slide is then mounted on the scale so that the dielectric coating on the slide (transmitter and receiver) electrodes is positioned adjacent to the dielectric coating on the scale (signal) electrodes. That is, placing the dielectric coatings between the signal electrodes and the transmitter and receiver electrodes minimizes these adverse effects. In addition, the dielectric coating on the slide slidingly contacts the dielectric coating on the scale. The sliding contact between the dielectric coatings reduces the gap between the slide and the scale into which the contaminants intrude.
The sliding contact approach requires that the electrodes be resiliently biased toward each other. The resilient bias accommodates deviations from exact surface flatness and alignment by permitting the electrodes to move apart from each other. This allows the dielectric layers to be forced apart from each other. Thus, when using such a capacitive position transducer in a highly contaminated environment, contaminants can force the slide away from the scale, collecting between the slide and scale. Thus, this approach is inadequate under some circumstances.
However, using thick dielectric coats, rather than the thin coats taught by Gerhard et al., has reduced, to some extent, the negative effects due to contaminants collecting between the slide and scale. The thick dielectric coats create a pair of capacitors connected in series with the capacitance created by the contaminants. The capacitance created by the dielectric coats does not vary as the slide moves along the scale. Thus, the change in capacitance between the signal electrodes and the transmitter and/or receiver electrodes resulting from changes in the thickness or the composition of the contaminants is dominated by the fixed capacitances created by the thick dielectric coats. Although using thick dielectric coats can reduce the problem caused by dielectric contaminants, this approach cannot completely eliminate the problem.
Another approach isolates the electrodes from the liquid and particulate contaminants. For example, the capacitive position transducer linear scale may be sealed. However, sealing the linear scale increases the fabrication and assembly costs and is often unreliable. Also, such seals are difficult to practically apply to all sizes and applications of electronic linear scales.
Magnetic transducers are alternative types of position measuring transducers. Magnetic transducers are relatively insensitive to contamination caused by oil, water and other fluids. Magnetic transducers, such as the Sony Magnescale encoders, employ a read head that detects magnetic fields and a ferromagnetic scale selectively magnetized with one or more periodic magnetic patterns. The read head senses changes in the magnetic field as the read head moves relative to magnetic scale patterns on the scale. However, magnetic transducers themselves are affected by small particles, particularly ferromagnetic particles attracted to the magnetized scale. Consequently, magnetic transducers must also be sealed, encapsulated or otherwise protected to keep contaminants from affecting their accuracy. Magnetic transducers also do not offer the very low power consumption desired for some applications of electronic linear scales.
Inductive transducers, in contrast to both capacitive and magnetic transducers, are highly insensitive to cutting oil, water or other fluids as well as to dust, ferromagnetic particles, and other contaminants. Inductive transducers, such as the INDUCTOSYN.RTM. type transducers, employ multiple windings on one member to transmit a varying magnetic field received by similar windings on another member. The multiple windings can be a series of parallel hairpin turns repeated on a printed circuit board. An alternating current flowing in the windings of the first member generates the varying magnetic field. The signal received by the second member varies periodically based on the relative position between the two members. A position determining circuit connected to the varying signal from the second member can determine the relative position between the first and second members. However, both members are active. Therefore, each member must be electrically coupled to the appropriate driving circuitry. This coupling is typically achieved by wired connections. When a moving wire connection must be attached and routed for free movement, the reliability of the unit is generally lower while the maintenance cost and production cost of the unit are higher. Sometimes an electrical coupling approach based on transformers is used in place of wire connections to the scale elements. This approach may reduce signal quality and accuracy, and may require additional size and cost to accommodate the placement of coupling elements on the scale and read head. Moreover, such a system would usually consume more energy than can be practically provided with conventional batteries, reducing the portability and the ease of installation.
Other motion or position transducers that are insensitive to contaminants are described in U.S. Pat. No. 4,697,144 to Howbrook, U.S. Pat. No. 5,233,294 to Dreoni, and 4,743,786 to Ichikawa et al., and British Patent Application 2,064,125 to Thatcher. These references disclose position detection devices that sense position between an energized member and an inactive or unenergized member. The transducing systems described in these references eliminate electrical intercoupling between the two moving members, a drawback of inductive transducers. However, these systems generally fail to provide the high accuracy of inductive or capacitive transducers.
Additionally, in some of these transducing systems, the inactive member is preferably ferromagnetic, to produce a sufficiently strong magnetic field. Ferromagnetic members offer limited fabrication options, and may become magnetized in the presence of magnetic chucks, thereby attracting magnetic particle. Alternatively, the inactive member is moved within a magnetic field defined and concentrated by a complex structure formed in or on the active member. The transducing systems disclosed in these references also produce output signals that are discontinuous or are not a simply prescribed function of position. Such signals contribute to inaccurately determined relative positions. Generally, all of these systems fail to provide the combination of sufficient accuracy and extensive measuring range that is commercially demanded for linear scales.